Nanostructures or nanoparticles are plentiful in nature and form the basic building blocks for chemical and biological compositions. Nanoparticles may also be created by artificial means, either chemical or mechanical, or both, to take advantage of property improvements associated with their use. Use of nanoparticles allows greater accessibility and availability of many components for certain applications and may reduce the amount of a component necessary to achieve a given result, thus reducing costs attendant with the use of the component. Small particle size is itself a necessary property for colloidal stability and for high performance of particle dispersions in some applications, including jetting.
Small particles, and in particular, nanoparticles, may be prepared either by reducing the size of larger particles or by constraining growth of particles as they are formed, or by a combination of techniques. For example, the size of larger particles may be reduced by any number of mechanical or physical techniques known to those skilled in the art. These techniques include, without limitation, the application of energy through milling, ultrasound or high sheer mixing, such as, but not limited to, a media mill, ball mill, an attritor, a flow jet mixer, an impeller mill, a colloidal mill, or a sand mill. Alternatively, smaller particles may be formed during synthesis by constraining their growth, for example, by formation in a micro-channel reactor. Finally, particle size may be reduced by dissolving larger particles and constraining growth during recrystallization. This may be accomplished, for example, by precipitating the particles from solution in the presence of surfactants, among other methods known in the art.
More recently, it has been reported that nanostructured particles of inorganic minerals have been formed in lyotropic liquid crystals having hydrophilic and hydrophobic domains within the crystal. This method is not used commercially.
The prior art techniques for creating small particles are not without shortcomings. Typically, the most effective commercial techniques to obtain small particles, including nanoparticles, require reduction of the size of larger particles, accomplished by the application of mechanical or physical energy or constraining particle size growth, as discussed above. Both of these approaches require highly specialized equipment and are time consuming, and both the equipment and processes are expensive.
In addition, smaller particle sizes are generally associated with larger surface areas, and nanoparticles are no exception. Due to their larger surface areas, among other things, nanoparticles require stabilization to prevent agglomeration and maintain their dispersibility in suitable media, making them more accessible or available for their ultimate use. Hence, following reduction of particle size, it is typically necessary to stabilize the nanoparticle dispersion through a separate step.
Colloidal dispersions of small particles, including nanoparticles, may be stabilized by several different techniques, including without limitation i) the addition of polymeric or small molecule surfactants that associate non-covalently with the surface of the particle, ii) through covalent attachment of “stabilizing” small molecules, or iii) polymers to the surface of the small particle, or by encapsulation of the small particle with components that will contribute to the stabilization. Encapsulation may be accomplished, for example, by cross-linking polymeric surfactants or polymerizing monomers, which are then adsorbed to the surface of the particle.
Some stabilization examples from the prior art include U.S. Pat. No. 7,741,384, which is directed to a method of homogenizing a dispersion by coating pigment particles with a polymerized monomer. Similarly, U.S. Pat. No. 7,307,110 describes methods for improving dispersibility of a water-based pigment by treating the surface of the pigment particle with a water-dispersibility-imparting group or encapsulating the pigment particle with a water-dispersible polymer. U.S. Pat. No. 6,432,194 describes methods of attaching functional groups to pigment particles to improve various properties rather than relying on adsorption. U.S. Pat. No. 6,171,381 is directed to an aqueous ink composition wherein cyclodextrin is used as a coating agent; dextrins are also used as dye binders in KR 100258640.
Prior art stabilization techniques involving the addition of surfactants, covalent attachment of “stabilizing” small molecules or polymers to the surface of the particle, or encapsulation of the particle, while useful, are not without disadvantages. Surfactants may change the properties of the dispersion in undesirable ways, such as by increasing viscosity or lowering surface tension, and they may also be expensive. Practical commercial techniques to stabilize small particles by covalent attachment of small molecules or polymers and/or by encapsulation tend to require relatively complex, multi-step chemical processes and may use undesirable or dangerous solvents or reagents. There is, therefore, a need for a process for preparing stabilized nanoparticle dispersions that allow for accessibility and availability of the nanoparticle component in the selected application, without compromising the properties of the dispersion and that are simple and cost effective to produce.
Novel self-assembled nanoparticles and unique processes for preparing them have been discovered, which avoid the shortcomings of the prior art discussed above. The novel self-assembled nanoparticles of the invention are clathrates formed by the addition of a selected guest solid to a structured fluid or matrix, i.e., a semi-solid or viscoelastic gel comprising a host vessel or molecule dispersed in an acid or other solvent medium. The host vessel may comprise a number of compounds known to one skilled in the art to be useful as host molecules in supra-molecular chemistry. These include: native or modified polysaccharides; cavitands, such as cyclodextrin, cucurbituril and calixerenes; simple sugars, such as dextrose, fructose or glucose; simple (linear, branched, or cyclic) polyols, such as ethylene glycol, propylene glycol, glycerin, sorbitol and xylitol; crown ethers, aza crowns, cryptands, cyclophanes, oligo- and poly-peptides, proteins, oligo- and poly-nucleotides, or other similarly structured molecules. The selected guest solid is entrapped or otherwise included within the host vessel to form a clathrate cage or shell having the selected solid (guest) encompassed within. The particle size of the selected solid is thus reduced or growth is limited by the structural constraints of the host molecule.
Clathrate or host/guest formations are known in the art, although none of the prior art describes the specific clathrates of the present invention, or processes, for reducing particle size of a selected solid to nanoparticle dimensions through the use of a clathrate and/or stabilizing a colloidal dispersion of nanoparticles, which do not require additional particle size reduction or stabilizing steps. For example, U.S. Patent Publ. No. 2004/265237 discloses a small molecule clathrate useful for improving the solubility and release of platinum based anticancer drugs, but the disclosed clathrate is not a nanoparticle-based clathrate. Similarly, U.S. Pat. No. 6,881,421 discloses a nano-polyalkylcyanoacrylate plus an inclusion compound useful for complexing an “active” in its hydrophobic cavity, useful as a drug carrier. U.S. Pat. No. 7,462,659 discloses uniform nanoparticles useful as pore-forming templates on wafers of electronic material, wherein cyclodextrin is combined with silica to form a low dielectric film. U.S. Pat. No. 7,829,698 describes nanoparticles comprising cucurbituril derivatives and pharmaceutical compositions in THF organic solvent for use as a drug delivery system.
With respect to inks and jetting applications in particular, none of the prior art discloses the novel nanoparticle-based aqueous colloidal dispersions of the present invention. JP 2001271012 describes a nanoparticle-based ink formulation prepared by first mechanically reducing the particle size of the pigment and combining the pigment with a number of components including amides, polyhydric alcohols, urea, glycerin, glycols, ethers, buffers, and water. Cyclodextrin or calixarene are added to aid in dispersibility and stability of the formulation in the same manner as surfactants.
By contrast, the present invention does not require or utilize reduction of the particle size of the selected solids to nanoparticle dimensions prior to addition to the gel. Rather, reduction of selected solids to nanoparticles is accomplished in a one-step mixing process involving the addition of the selected solid to a gel comprising a host vessel or molecule dispersed in an acid medium or other solvent. Particle size reduction is accomplished by dissolving and reforming the solid in the host vessel, or by synthesizing the solid directly in the host vessel and annealing the interaction. Mechanical particle size reduction might be used prior to combining the selected solid with the gel, but only for particularly large particles or agglomerates to facilitate further reduction to nanoparticles using the inventive process. The invention yields stable colloidal dispersions without the addition of other steps or components.
CA2181495 discloses a water-based printing ink comprising an epoxy, an organic or inorganic pigment, a drier, cyclodextrin and water. Cyclodextrin forms an “inclusion compound” with the drier to protect it and to reduce the amount needed in the ink. Unlike the present invention, the cyclodextrin is not used as a host for the pigment, nor is it stated to reduce the particle size of the selected pigment.
U.S. Pat. Nos. 7,371,456 and 7,030,176 disclose new recording inks with improved properties comprising nanoparticles with colloidal inner cores used as a template to bind a series of layers of colors and a complex process for preparing them. The inks include optional “includant” compounds that may inhibit aggregation of the colors or add to the stability of the inks and cyclodextrin is listed as one such compound. Stability is primarily accomplished by charges on pre-formed polymers. Unlike the present invention, the inks require alternating layers of polymers and/or charged polymers to wrap or attach to colorants. The inks are formed in an oil/water system by high sheer emulsification, using organic solvents. In addition, preparation of the inks starts with a charged nano-particle core of either a charged polymer or a charged silica gel particle. Nothing in either of these patents teach the use of includant compounds to reduce particle size or stabilize the formulation.
The present invention is also directed to novel processes to achieve the novel self-assembled nanoparticles, colloidal dispersions thereof, and colloidal stabilization in a single step mixing process that is safe and environmentally friendly. The inventive processes involve the use of simple techniques to prepare nano-structured particles and stable colloids of these particles that may be easily practiced in, and are viable for, commercial manufacturing. The novel processes are also less costly, because they do not require specialized or additional equipment or steps, specialized handling or additional components.
Nano-structured particles prepared by the inventive processes have many valuable uses, among them are as stable colloidal dispersions useful for application by jetting technology. Stable, colloidal dispersions of organic color pigments have been prepared using this technology. These dispersions have been used to prepare inks with excellent jetting properties, although the invention is not limited to this application. Other uses of the novel, stable colloidal dispersions of the present invention include, but are not limited to, other types of inks and coatings; preparation of stable colloids of electronic materials such as conductors, insulators, semiconductors, and the like, particularly those useful for devices or manufacturing as by jetting; preparation of stable dispersions of organic and ceramic materials for various other applications; preparation of stable dispersions for biotechnology, pharmaceutical, drug delivery, medical diagnostics or bioassays, or imaging applications; and nano-fabrication of devices. Other uses will be evident to one skilled in the art.
The novel, stable colloidal dispersions of the invention have comparable particle size and comparable or better stability than those produced by traditional competitive processes and have demonstrated utility in jetting applications.
It is an object of the invention to provide a simple, one step method for reducing the particle size of a large variety of solids to nanostructured particles.
It is a further object of the invention to provide stabilized colloidal dispersions of nanoparticles from a wide variety of selected solids.
It is yet another object of the invention to provide commercially viable techniques for producing stabilized colloidal dispersions of nanoparticles of a wide variety of selected solids, which are simple, safe, cost effective and environmentally friendly to perform.